B. J. Bluck & J. K. Ingham comment: The status of the fossiliferous rocks at the Highland Border (the Highland Border Complex) has been a matter of speculation for over a century. The question has been whether the Highland Border Complex has affinities with the Dalradian block or constitutes a separate block, a sliver or slivers of which have been caught up along the Highland Boundary Fault.

A different approach to the naming of trace fossils is advocated. The primary ichnotaxobase should be the form of the burrow actually occupied, and the secondary ichnotaxobase should be the morphology of the structure that reflects the manner in which this burrow has been displaced and/or extended. Only by attempting to name trace fossils in this way will it be possible to eliminate features due to sedimentological factors that took place on termination of the animal’s activities, including passive infill and diagenesis. To discriminate between different preservational states the citation should include both the taxonomic and preservational aspects.

All Cambrian series and several Cambrian biozones have been recognized using acritarch biochronology within the siliciclastic successions underlying Upper Silesia in southern Poland. The entire Cambrian succession is around 580 m thick and contains rare Lower Cambrian trilobites of the Acado-Baltic faunal province. Acritarch associations are taxonomically comparable to those recorded in Baltica, Laurentia and Gondwana, but their closest taxonomic affinity is with Iberia. The Cambrian succession accumulated in a shallow shelf environment and is almost flat-lying, unmetamorphosed, uncleaved and in normal stratigraphic order. It underlies paraconformably Lower Devonian deposits and overlies unconformably steeply dipping metasediments of undetermined Precambrian age. Tectonic deformation and metamorphism to greenschist grade in these Precambrian strata must have occurred in the Proterozoic, and are attributed to the Cadomian orogeny because similar Cadomian basement complexes occur in the adjoining Brno Massif and in the East Avalonian and Armorican terranes. Upper Silesia appears to be a stable crustal block bordered by deep faults whose sedimentary cover has not been affected by tectonic deformation other than faulting. Based on the recognition of Cadomian age basement, the distribution of trilobites and acritarchs and the tectonostratigraphic relationships to adjacent areas, the Upper Silesia terrane is interpreted to be a distal segment of East Avalonia that in Cambrian times faced Iberia. An extension of the Tornquist Suture from the Intra-Sudetic Fault is seen in the Kraków-Myszków Fault Zone at the margin of Upper Silesia. The Intra-Sudetic Fault zone and the Kraków-Myszków Fault Zone contain Early Palaeozoic rocks deformed during the Caledonian orogeny, and mark the boundary between the Caledonian accretionary belt and areas unaffected by this orogeny.

Two metasedimentary complexes are exposed in the Sudetes Mountains of Poland, Germany and the Czech Republic. Succession I comprises pelites and greywackes of Neoproterozoic–Cambrian age, deformed and metamorphosed prior to intrusion by S-type porphyritic granites at 515–480 Ma. Succession II comprises a sandstone–mudstone–chert sequence and turbidite sequence of Ordovician–early Carboniferous age accompanied by bimodal volcanogenic rocks. Both successions were intruded by late- to post-orogenic granitoid intrusions at 340–300 Ma. The sedimentary rocks of succession II show increasing maturity until mid–late Devonian times. Inversion of the basins, commencing in late Devonian–early Carboniferous times, was reflected in the emplacement of turbidites and olistostromes, concurrent with the uplift of a metamorphic core complex of succession I rocks. The original stratigraphic order of the successions was maintained, thus crustal imbrication was not significant. Instead, extensional faulting became important, followed by transpression on almost orthogonal fault zones, resulting in the presently observed juxtaposition of crustal blocks. The Palaeozoic sequences developed in mainly ensialic basins on Cadomian and older basement, parts of which became strongly reworked (2.6 to 0.54 Ga zircon inheritance ages) and incorporated into the Palaeozoic structures. The orogen developed either on the rifted margin of peri-Gondwana or on a rifted-away fragment of pre-Baltica. The Sudetic section of the Variscan Orogen is of broadly Alpine style, with significant basement involvement, but apparently without evidence for long-lived subduction of wide oceans or the accretion of numerous exotic terranes.

M. J. M. Turnbull & S. Moorbath comment: The paper of Yang et al. (1996) describes a novel dating approach, using the Sm–Nd method on phosphatic material, to determine the age of the Precambrian–Cambrian boundary. The dates reported are of a precision which, in light of the relatively small range of 147Sm/144 Nd ratios (0.09–0.17), is surprising. We have recalculated the published data using the ISOPLOT program of Ludwig (1991) and this is shown below in Table 1. These results cast considerable doubt on the statistical analysis of Yang et al. (1996). While this technique may have utility with Precambrian successions it would appear to be of limited use for the high precision dating required for Phanerozoic stratigraphic correlation.

The mafic metavolcanic rocks (blueschists and greenschists) of the Rýchory Mountains crystalline complex (West Sudetes) experienced sea-floor hydrothermal alteration (spilitization?) prior to regional metamorphism. The metabasite geochemistry (namely trace element and REE abundances) indicates that the protolith was comparable in composition with (1) tholeiitic to transitional ocean-floor basalts and (2) transitional and alkaline intra-oceanic island basalts. Two main metamorphic events affected the Rýchory Mountains metabasites. In an earlier high pressure–low temperature metamorphic event, the rocks experienced blueschist facies metamorphism. The results of 40Ar–39Ar geochronology studies on phengites from the mafic blueschists date the end of the earlier metamorphism to 360 Ma. The greenschist metamorphic overprint followed around 340 Ma. The elongated bodies of mafic metavolcanic rocks are situated within the prominent NNE–SSW Leszczyniec shear zone following the trend of the Rýchory Mountains and the Rudawy Janowickie Mountains. Both the geochemical affinities and the blueschist facies metamorphism of the metabasites suggest that this shear zone evolved from the Variscan suture dividing western and central terranes of the West Sudetes. According to the radiometric age for the end of the high pressure–low temperature metamorphism, the terranes accreted during the Famennian. A considerable time-span between the formation of the metabasite protolith and the blueschist metamorphism may indicate long-lasting subduction of a large oceanic plate between Gondwana and Laurussia, possibly accompanied by terrane accretion, prior to the Variscan orogeny.

In the Sudetes, seven distinct lithostratigraphic terranes exhibit a symmetric distribution. A central region of basinal/oceanic and ophiolitic rocks, the Central Sudetic terrane is bordered, respectively to the northwest and southeast, by the sialic Saxothuringian and Moldanubian terranes. These exhibit contrasting metasedimentary/metavolcanic successions and tectonic-metamorphic sequences, but both are characterized by Palaeozoic plutonism. These are in turn bordered (again respectively to the northwest and southeast) by the Lusatian and Moravian terranes, which are also sialic, but contain Cadomian granitoids and represent rifted and now widely separated fragments of Gondwana. Along the southwestern flank of the Sudetes, the Barrandian terrane, largely covered by younger sediments, extends to the southwestern margin of the Bohemian Massif. The Sowie Góry terrane forms a klippe of high grade gneisses tectonically emplaced on top of low-grade, sheared ophiolites of the Central Sudetic terrane. The Sowie Góry terrane exhibits a history of three distinct, probably multi-orogenic, regional metamorphic events: an early high-pressure granulite/eclogite metamorphism followed by medium- to low-pressure granulite, and in turn by amphibolite facies metamorphism. All the terrane boundaries are complex zones of ductile to brittle shearing, modified by later brittle movements. Some, such as the Leszczyniec shear zone, mark lines of old, pre-Variscan rift and suture zones, reactivated and overprinted during a series of Variscan ductile to brittle events of extensional shearing with related metamorphism and plutonism.